Miniature, low cost, small-signal transformers for impedance matching and conversion of single-ended to differential (BALUNS) are sometimes prohibitively large for portable designs using standard wire-wound core technology. Some advances in low temperature co-fired ceramic ferrite tapes and pastes allow fabrication alternatives to wire-wound cores. For example, some fabrication processes for a transformer structure or similar device use metallized magnetic substrates or green tape processes, such as disclosed in U.S. Pat. Nos. 6,007,758 and 5,802,702. For example, vias can be formed through a ceramic body and sidewalls coated with a conductive material. An aperture can be formed through the ceramic body and intersect the via. The unfired ceramic body can be metallized such that a conductive pathway is formed. Also, some devices can be formed from multiple unfired ferrite layers a single via coating step, permitting green tape-type fabrication.
Other processes use traditional low temperature co-fired ceramic (LTCC) and ferrite tape/ink combinations, such as disclosed in U.S. Pat. Nos. 5,312,674 and 5,532,667. For example, a ferromagnetic material can be provided in ink or tape form and sinterable, using a firing profile that is about the same thermal shrinkage characteristics as low temperature co-fired ceramic tape.
Other magnetic components can be fabricated as monolithic structures using multilayer co-fired ceramic tape techniques such as disclosed in U.S. Pat. No. 5,349,743. Multiple layers of a magnetic material and an insulating non-magnetic material can form a monolithic structure having magnetic and insulating non-magnetic regions. Windings can be formed using screen-printed conductors connected through the multilayer structure by conducting vias.
Improvements are still desired to ensure that traditional thick film printing and commercially available multilayered ceramic (ferrite) tape processing can be used with silver and gold thick film conductors without wire winding. It is desirable that small designs be implemented for high frequency, small-signal applications having a low profile. Flexible designs are desirable that allow the conductor and core to be integrated. A minimum number of layers is desired with a simple pattern to provide a tightly coupled interaction between primary and secondary windings.